Equalizing vacuum-tube amplifier



Nov. 10,1925. 1,561,279

M. l. P UPlN EQUALIZING VACUUM TUBE AMPLIFIER Filed Dec. 23, 1918 2 Sheets-Sheet l I nveyvfo'e W X/Lani, Q. gwy

Nov. 10, 1925- M. l. PUPIN EQUALIZING VASJUUM TUBE AMPLIFIER Filed Dec. 23, 1918 2 Sheets-Sheet 2 Y mi Pia ze ra/zg Patented Nov. till, 19.25..

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MIOHAEL I. PUPIN, OF NORFOLK, CONNECTIOUT, ASSIGNOR T0 WESTINGHOUSE ELEC- TRIO AND MANUFACTURING COMPANY, OF EAST PITTSBURGH, PENNSYLVANIA, A

CORPORATION PENNSYLVANIA.

EQUALIZING VACUUM-TUBE AMPLIFIER.

Application filed December 23, 1918. Serial No. 268,044.

To all whomit may concern:

Be it known that I, MICHAEL 1. Porn, a citizen of the United States, residing at Norfolk, in the county of Litchfield, State of Connecticut, have lnvented certain new and useful Improvements in Equalizing Vacuum- Tube Amplifiers; and I do hereby declare the following to be a full, clear, and exact description of the invention, such as will enable others skilled in the art to which it appertains, to make and use the same.

This invention relates 'to vacuum tube amplifiers, and involves a method and apparatus by means of which the amplifying )owerof vacuum tube amplifiers are equal- .lzed, so that these tubes can replace each other, particularly in a balanced multi-step am lifier system.

ommercial vacuum tube amplifiers, while seemingly identical, often, and generally, have slight structural. inequalities which affect their amplifying action or power. The amplifying actions of such commercial tubes thus generally difi 'er, so that it is usu ally impracticable to directly replace, in any balanced amplifying system, one vacuum tube by another. The aim of the present invention is to provide a method of and means for equalizing the amplifymg action of such tubes, so that one tube can be readily and conveniently replaced by another tube, without unbalancing the system. In carrying out the method of the invention, 1 first determine the amplifying action of each of the tubes which it is desired to equalize. This determination I prefer to make by use of a wave balance, such as described in my copending application for Letters Patent of the United States, Serial No. 257,570, filed October 10, 1918, upon which Patent No. 1,456,909 was granted Ma 29, 19.23. These determinations, preferab y visualized by suitable curves, indicate the magnitude and character of the inequalities in amplifying action of the various tubes. I then relative- 1y adjust, taking one tube at a time, the potentialrelations of the electrodes of these tubes, until the amplifying actions of all of the tubes over the desired range of operation, are substantially equivalent. By de terminin and noting the adjustment made with each tube to produce the desired amplifying action, 1 am able to predetermine, in the case of each tube, just what adjustment must be made in the potential relations of its electrodes whenever one tube is to be replaced by another in a balanced amplifylng system.

graduating the same, and by using it in equalizing the various vacuum tubes under such conditions of. operation as normally prevail in the system, the vacuum tubes may be calibrated in terms of the graduations of the resistance, so that in replacing one tube by another, it is only necessary to adjust the resistance as indicated by the calibrationof the tube to be'placed in the system. 7

o The novel features of the invention which I believe to be patentable are definitely set forth in the appended claims. These features, together with the arrangement and mode of operation of apparatus embodying the same, will be best understood from the following description, taken in conjunction with the accompanying drawings, in which; Fig. 1 represents diagrammatically an arrangement of apparatus, for modifying the amplifying action of-a vacuum tube amplitier; Fig. 2 represents diagrammatically the use of a wave balance for determining the amplifying action of a vacuum tube amplitier connected as indicated in Fig. 1; Fig. 3

diagrammatically represents one unit in a balanced amplifier system in which the equalizing apparatus of the present inven-' tion is employed and Fig. 4 illustrates certain curves which representthe amplifying actions of three vacuum tube ampiifiers.

Referring to Fig. 1 of the drawings, there is diagrammatically represented a low im' pedance vacuum tube amplifier having a grid electrode 10, a hot electrode or heating filament 11 and a plate electrode 12. A local source ofdirect current electric energy, such, for example, as a battery 13, serves to supply the relatively low voltage, say about seven volts, for maintaining a filament current of about 1.3 amperes, and the relatively high voltage, say about 100 volts, for the wing or electron current between the filament 11 and the plate 12.

The plate electrode 12 is connected to the positive terminal of the battery 13 through a suitable inductance coil 14. One terminal of the filament 11 is connected to the positive terminal of the low voltage section of the battery 13, While the other terminal of the filament is connected through a resistance or rheostat 15 to a common ground connection. 16. The heating current for the filament 11 thus passes through the conducting wire of the resistance 15; a

The resistance 15is provided with a scale graduated in suitable units. A sliding or adjustable contact 17 serves to connect any pointcof the resistance 15 to the grid 10, and thus places the grid at any desired potential below the potential of the filament.

As is well known, the lower the potential of the grid with respect to the potential of the filament, the higherwill be the effective internal impedance of the vacuum tube and, the external impedance and the plate potential being fixed, the lower will be the amplifying power of the tube. It is obvious then that it the potential. of the plate electrode is fixed, say 100 volts, and the heating filament current, as well as the other elements in the structure represented in Fig. 1, are fixed, then the motion of the movable contact 1?, by varying the potential of the grid electrode 10, will vary the amplifying power of the tube. in this manner, the amplifying action of a vacuum tube can be rery readily modified, in order to render its amplifying power substantially equivalent to the amplifying power of a second vacuum tube.

Fig. 2 of the drawings diagrammatically represents the procedure described in my aforementioned copending application for using a wave balance in determining the amplifying action of a vacuum tube arranged as represented in Fig. 1. Corresponding elements of these two figures are indicated by the same reference numerals. The wave balance comprises inner and outer laminated layers of tin-foil 18 and 18' respectively, between which is positioned a coiled wave conductor '19, separated by Suit able (ii-electric layers from each tin foil layer. The W&Y6 CQI1dllCtQ1-h&S substan: tially uniformly distributed inductance and ca acityand possesses su-fficiently high re- 51 ance and capacity to make it a true wave conductor and to render it practically a pcriodic. The secondary coils 20 and 21 of the wave balance are 1nductively associated with the wave conductor 19 and are relatively movable with respect to each other and to the wave conductor. The foregoing elements of the wave balance are diagrammatically indicated in Fig. 2 of thedrawm s.

The beginning of the wave conductor 19 is connected to one terminal of a source of alternating electromotive force while the end of'the wave conductor is connected to the common ground 16. The terminals of the secondary coil 20 are connected. by a suitable resistance 23. This resistance should be large in comparison with the impedance of the secondary coil. The resistance 23 has an adjustable contact or terminal 24, and functions in effect as a noninductive potentiometer. The adjustable contact 24 is connected to the grid 10. One terminal of each of the secondary coils 20 and 21 is connected to the common ground 16. The ungrounded terminal of the sec.- ondary coil 21 is connected to the plate electrode 12 through a condenser 25 and the primary winding of a transformer 26. The secondary winding of the transformer '26 is connected to a heterody'ne receiver, indicated generally by reference'numeral 27. By means of the adjust-able contact of the potentiometer 23, a.suitable fraction of the voltage induced in the secondarycoil 20 is impressed upon the grid 10 and is reproduced in amplified form at the plate 12, and will there produce an alternating current through the primary winding of the trans former 26, condenser 25, secondary coil 21 to ground and back to the plate 12. TVhen, however, the clectro-motive force induced in the secondary coil 21 is of opposite phase and of equal amplitude, no current will flow in the'circuitjust mentioned, and the heterodyne detector will be silent. This silence is, however, produced by adjusting the amplitude of the electro-motive force impressed upon the grid 10 and by moving the secondary coil 21 until the phase relation is properly adjusted so as to produce silence in the detecting telephone. The am lifying power of the vacuum tube ampli or is then calculated from the potential impressed upon the grid and from the osition of the secondary coil 21 by means 0 the attenuation curve of the wave balance for the frequency under consideration.

In accordance with the present invention, the amplifying power of each of the tubes which it is desired to e ualize, is measured in the manner describe in thepreceding paragraph. In this manner, the three curves represented by A, B and C in Fig. 4;, were obtained- These three curves represent what may be termed the characteristic ainplifying curves of three low impedance lid ' vacuum tubes A, B and The amplifying power of each tube was measured by the wave balance, as hereinbefore described, for various potentials from 150 volts .of the plate electrode 12. In determining the amplifying power .of these three tubes, an arrangement of apparatus such as illustrated in Fig. 2 was employed, and the movable contact 17 ,was maintained, in each case, on scale division 21. Each of the three curves A, B and C of Fig. 4, thus bear the legend SD 21. The filament current in these tubes was normal; that is, 1.3 amperes. The total resistance of the rheostat or resistance 15 was 3.5 ohms, so that at scale division 21, which is somewhat less than one-quarter of the whole rheostat are, the potential difference between grid and filament was approximately .95 volts. 7

Curves A, B and C clearly show that the amplifying powers of these three tubes differ very considerably under the conditions specified hei'einbefore, and, therefore, they cannot replace one another in a balanced multi-step amplifier, such, for example, as described by the applicant in his co-pending application Serial No. 257,571, filed October 10, 1918, upon which Patent No. 1,452,933 was granted April 21, 1924. \Vhat is true of these three tubes, which were selccted at random, is generally true of all vacuum tube amplifiers.

Curves A, B and C of Fig. 4 were obtained for the same threevacuum tubes, in the manner diagrammatically indicated by Fig. 2, with the movable contact 17 suitably adjusted, in each case, to render the amplifying action of the three tubes, over a desired range of operation, as nearly equivalent as possible. Thus, curve A was obtained by placing the movable contact 17 at scale division 99. which put the grid of the tube A at relatively low potential with respect to the filament and thus diminished the amplifying power of this tube. The characteristic amplifying curve of this tube was thus transformed from curve A to curve A. In a similar manner, the characteristic amplifying curves for the other tubes Band C were transformed into the curves B and C by contact 17 at scale division 66 and' atscale division 25, respectively. The three curves A, B and (1 coincide at the plate poten tial of volts and for a considerable interval above and below 100 volts. For that interval these tubes can replace each other. Some other interval can be selected just as well. l have found that by far the greatest number of reasonably good tubes of this type can be made equivalent to each other by adjusting the voltage of the grid with respect to the volt-age of the filament (the so-called C voltage}, in the manner just described.

lacing the movable The manner of utilizing this method of equalizing the amplifying action of vacuum tubes is diagrammatically represented in Fig. 3 of the drawings. In this figure, the

vacuum tube and its associated parts, pre- The secondaries 20 and 21 of the wave balance are connected in series with an adjustable capacity 31 and a high resistance conductor 32. as described in my copending application, Serial No. 257,571, (Patent No. 1,452,933) aforementioned. ,The movable contact 17 connects a suitable point, the point of equivalency for the vacuum tube under consideration, of the resistance or rheostat 15; to the grid electrode 1.0 through th high resistance 32. \Vhen the amplifying system represented by Fig. 3 is adjusted according to the principles described in my copending application, last referred to, it will stay so adjusted when the vacuum tube amplifier 1011-12 is replaced by another amplifier made equivalent to it by moving the contact 17 to a predetermined point of the resistance 15, which operation adjusts the voltage of the grid 10.

From the foregoing description, it will be observed that my present invention involves a determination of the amplifying action of the vacuum tube amplifyer to be equalilzed under substantially similar predetermined conditions of operation, and. their an adjustment of the potential relations of the electrodes of these tubes until the anq'ilifying action of the tubes, over a desired range of operation, are substantially equivalent. and finally a determination of the character of such adjustments, so that the same may be reproduced at will. While I prefer, in producing the desired adjustment of the potential relations of the electrodes of the vacuum tubes, to I adjust the potential of the grid electrode with respectto the filament, other ways of effecting the desired adjustment of the electrode potentials may be used, as, for example, by inserting in the plate circuit a suitable-resistance, and by its variation varying the plate voltage. However, 1 prelUU fer to adjust the potential of the gridelectrode with respect to the filament be cause it affords the possibility of running a multi step amplifier in which each tube has the same plate voltage and in which the same filament current is obtained "for each tube by making the resistance of the various wing induetances (1%} the same, and the resistance of allrheostats (15) the same. In this manner, one voltmeter sufiices to indicate the voltage of each plate and one ammeter sulfices to control the currents in the various filaments.

I claim:

1. The method of measuring the amplifying power of a three-electrode vacuum tube which consists in deriving two periodic clectromotive forces of opposite phase from a common source of periodic energy, applying a measured adjustable fraction of one of said elcctromotive forces between the grid and cathode, applying the other of said electromotive forces between the anode and cathode," detecting "the presence of from an adjustable point in said resistance to the grid, which consists in deriving two periodic electromotive forces of opposite phase froma common vsource of periodic energy, applying a measured adjustable fraction of one of said electromotive forces between the grid and filament, applying the other of said electromotive forces between the anode and .filament, detecting the presence of periodic current in the anodecathode circuit, and adjusting said fraction amplifying power of the tube from said measurements, changing the adjustment of said point and repeating said measurements, doing the same with different plate voltages, selecting from the several sets of measurements the position of said point which gives the desired am lification power over a selected range of p ate voltage and adjusting said point to said osition.

In testimony whereof I a x my signature.

MICHAEL I. PUPIN. 

